1. Field of the Invention
The present invention relates to a method for controlling at least one piezoelectric actuator of a fuel injector for an internal combustion engine of a vehicle, said at least one piezoelectric actuator acting on a valve means to open or close said injector, respectively allowing or preventing the injection of fuel into a combustion chamber of the engine, said method being applied by an engine control unit on-board the vehicle in operation and comprising the following steps:                application to the piezoelectric actuator of a first nominal electrical charge necessary for opening of the injector, called the nominal control charge, according to a nominal current as a function of the torque required and the engine speed, so as to open the valve means of the injector for the injection of fuel into the combustion chamber,        application to the piezoelectric actuator of at least one second electrical charge, called the polarization charge, in addition to said nominal control charge, on said nominal control charge after application thereof and before a step consisting of commanding a closure of the injector, so as to polarize the piezoelectric actuator during an opening phase of the injector and during the injection of fuel into the combustion chamber,        commanding the closure of the injector so as to stop the injection of fuel by application to the piezoelectric actuator of at least one electrical discharge, so as to close the valve means.        
2. Description of the Related Art
It is known to use a piezoelectric actuator for controlling the opening and closure of an injector in an internal combustion engine operating by means of petrol or diesel fuel.
A piezoelectric actuator is in principle composed, in the known manner, of a ceramic stack defining a determined length, which has the property of modifying its length under the effect of an electrical field and conversely of producing an electrical field under the effect of a mechanical stress; this stack is arranged in an injector between a stop and a valve means and operates in brief as follows: on application of an electrical charge to the piezoelectric actuator by means of an electrical voltage, its length increases and opens the valve means of the injector which releases pressurized fuel into the combustion chamber. At rest, i.e. with the valve means in the closed position, a clearance exists between the piezoelectric actuator and the valve means, so as to guarantee closure of this valve means and avoid uncontrolled fuel leaks towards the combustion chamber.
In order to be stable and have a reproducible behavior, a piezoelectric actuator must be polarized to a reference value, called the initial polarization; this is done in the factory on production of said actuator before the engine is used in a vehicle. Such a polarization consists of applying to the terminals of the piezoelectric actuator an electrical charge via a determined voltage, called the polarization voltage, for a time also determined, the effect of which is to orient the crystalline structure of the latter according to the direction of the electrical field imposed on the piezoelectric stack, which corresponds to the direction in which the variation in dimension of the piezoelectric actuator is desired. After elimination of this initial polarization voltage at the terminals of the piezoelectric stack, the latter retains a residual polarization state for subsequent usage.
However, during use in an internal combustion engine, a piezoelectric actuator tends to lose this initial polarization, in particular due to essentially urban use of the vehicle which leads to low engine speeds and hence low nominal electrical voltages for controlling the injectors under a low fuel pressure, largely below the polarization voltage.
Also, the electrical charge necessary to control the opening of the injectors may be adjusted to prevent the noise created by this injector opening being audible above the general engine noise, i.e. a map of nominal voltages for opening the injectors is established in the factory as a function of the engine speed.
Other usage conditions such as repeated cycles of increase/reduction in engine temperature, or long periods without use of the piezoelectric actuator corresponding to periods of extended immobilization of the vehicle, may also over time lead to a change in the polarization of the piezoelectric actuator.
French patent application FR 1254719 filed on 23 May 2012, not published at the date of filing of the present application, in the name of the applicant, discloses a solution to remedy the drawbacks of the depolarization described above. This solution allows the piezoelectric actuator to remain polarized automatically from the time of leaving the factory during usage of the vehicle, without requiring stoppage of the engine, and thus allows an extension of the service life of said actuators while reducing the maintenance operations. It also allows an improvement in performance of the injector by reducing the spread of clearance between the piezoelectric actuator and the valve means, and finally allows better control of the quantity of fuel injected into a combustion chamber and hence optimized fuel consumption and emissions. The application according to this solution of at least one electrical charge to the actuator at two times, a first time for nominal control of the piezoelectric actuator as a function of the injection calibration, then a second time for its polarization, avoids the polarization operation causing engine noise and in particular injection rattle. The method according to this solution can advantageously be used in an engine control unit (ECU) of the vehicle, by means of software for implementing the method, for controlling all engine injectors. It should be noted that this method may be integrated in the ECU computer of existing vehicles by means of a simple modified software for control/adjustment of the piezoelectric actuators of the engine injectors.
Conventionally, piezoelectric injectors are voltage-controlled by the engine control unit using a constant maximum current for a given engine cycle and cylinder, for application of the nominal electrical charge for commanding the opening of the injectors. The injector is therefore controlled by modifying the electrical voltage applied at the terminals of its piezoelectric actuator as a function of the common fuel rail pressure supplying the injectors, in order to provide precisely the energy required for the actuator to overcome the rail pressure which varies as a function of engine speed. However, depending on the engine speed concerned, in particular at low speeds for example at idle and just above, the injector opening time is reduced because of the low quantity of fuel injected into the combustion chamber, and the time available for applying the polarization charge during the opening of an injector may therefore be very short, even to the point of preventing application of the entire polarization charge. The application of a polarization charge according to patent application FR 1254719 filed on 23 May 2012 still does not in fact allow a significant polarization voltage to be attained, in particular in cases of a short injector opening time.